Color Calibration Adjustment Using Effective Ink Drop Weight

Color calibration is typically a dedicated, off-production process during which a series of dedicated colorant patches are printed and measured with a color measurement device, where adjustments are subsequently made based on the measurements and previous reference measurements. The adjustments compensate for a variety of factors, including variations in ink drop weight, ink-substrate interactions, and variations caused by environmental factors. As a result, color calibration happens periodically, interrupts print production and results in color consistency degradation between calibration events.

Color calibration methods are usable only between print jobs, but may not be used for improving within-job color consistency. In addition, due to resources required for performing color calibration (e.g., a substrate, ink, time, operator attention), the frequency of color calibration may only be as high as may be practically accommodated.

More frequent or on-the-fly color calibration may use a color camera or scanner to collect data during a print job. Doing so requires dedicated colorant patches to be printed (e.g., in margins of a print job), but may not be applied to workflows where full-bleed printing is required. In addition, the accuracy of cameras and scanners is more limited than dedicated color measurement devices such as spectrophotometers and, therefore, may not be applicable to more color-critical applications. Color measurements refer to an effect of a colorant on a substrate in terms of the colorant's resultant color.

Measuring ink drop weight using an ink drop weight (IDW) sensor (e.g., a Hewlett-Packard (HP) Drive Bubble Detect ink drop weight sensor) at print time and cross-linking IDW with an effective ink drop weight (eIDW) measured using a spectrophotometer enables color calibration to be updated continuously and reduces the frequency of performing color-measurement based color calibration, which improves color consistency form all types of printing from in-job via reprint to fleet and reduces waste due to print-based color calibration. IDWs are measured using an IDW sensor since IDWs concern colorant quantities, while eIDWs are obtained using a spectrophotometer since eIDWs concern an effect of colorants on a substrate in terms of a resulting color.

Examples described herein provide a device for and a method of determining an actual drop weight, determining an effective drop weight, and cross-linking the actual drop weight and the effective drop weight to adjust color calibration continuously in real-time and reduce a frequency of color measurement based color calibration. Thus, improving all types of color consistency from in-job via reprint to fleet and reducing waste due to print-based color calibration.

Examples described herein provide a device for and a method of adjusting color calibration where drop weight is measured using an IDW sensor and effective drop weight is measured using a spectrophotometer.

Examples described herein provide a device for and a method of inferring ink drop volume from in-printhead sensing or near-printhead sensing, where either type of sensing may occur at print time. Since drop volume measurement alone may not account for color changes, drop volume measurement, by itself, may not be directly suitable for color calibration.

Examples described herein provide a device for and a method that employ a combined approach to color calibration, where color measurement and drop weight detection are periodically cross-linked, which enables color calibration adjustments to be inferred from direct drop weight measurements and real-time color correction to be made without requiring dedicated calibration chart printing.

Examples described herein provide a device for and a method of improving various types of color consistency typically aided by color calibration (e.g., reprint and fleet consistency) and color consistency within single, long print jobs, which were previously beyond the reach of conventional color calibration in inkjet printing.

Examples described herein provide a device for and a method of keeping color calibration up to date during long runs or at higher frequencies than current print-measure solutions allow for.

1 FIG. 100 100 101 103 101 105 101 107 101 103 105 is a schematic of an example color calibration adjustment systemof the present disclosure. In an example, the color calibration adjustment systemmay include a printing system, an ink drop weight sensorconnected to the printing system, a spectrophotometerconnected to the printing system, and a processorconnected to the printing system, the ink drop weight sensor, and the spectrophotometer. In an example, a printing system may include a printer. In an example, a printer may include an inkjet printer, a scanning inkjet printer, and a page-wide array printer.

101 103 101 103 101 101 103 In an example, the printing systemincludes at least one printhead. The ink drop weight sensormeasures an ink drop weight (IDW) of each printhead of the printing system. The ink drop weight sensormay be located separately from (e.g., near) a printhead of the printing systemor in a printhead of the printing system. In an example, the ink drop weight sensormay be a Hewlett-Packard (HP) Drive Bubble Detect ink drop weight sensor.

105 101 101 105 101 The spectrophotometermay be located separately from the printing systemor in the printing system. The spectrophotometermeasures color printed by the printing system.

107 107 101 101 101 107 105 103 105 101 The processorincludes a color calibration method based on effective ink drop weight correction. For example, at calibration time, the processordetermines an effective ink drop weight and modifies the output of the printing systembased on changes in the effective ink drop weight between a previous reference state of the printing systemand a current state of the printing system. In an example, a change is between two ink drop weights on a single substrate type. The processordetermines an effective ink drop weight (eIDW) from color measurements made by the spectrophotometer. Thus, IDW measurements of the ink drop weight sensorare used to emulate the eIDW determined from color measurements made by the spectrophotometer. An IDW represents a weight of an ink drop from a printhead of the printing system. An eIDW indicates an effect of an ink drop on a specific substrate. For example, a 10% change in an IDW may correspond to a 7% change in an eIDW on one substrate but a 12% change in the eIDW on another substrate due to a difference in ink-substrate penetration, spreading, etc. between the two substrates. To maintain color consistency on a substrate, the effect of an ink drop on the substrate must be kept constant. Thus, a change in an IDW must be translated to a specific and different eIDW on a substrate-by-substrate basis.

2 FIG. 200 is a flow chart of an example color calibration adjustment methodof the present disclosure.

201 200 201 At block, the methodbegins. At block, a first set of ink drop weights (IDWs) and a first set of color measurements are determined for a first color calibration chart printed by a printing system using a first predetermined ink drop weight. The first color calibration chart includes first colorant ramps including first colorant patches with increasing levels of colorant coverage between adjacent first colorant patches for one or more colorants used in the printing system. The first set of IDWs (e.g., one IDW for each nozzle in the printing system, where a printhead may include one or more nozzles) may be measured using an IDW sensor while printing the first colorant patches of the first color calibration chart.

203 At block, a second set of IDWs and a second set of color measurements are determined for a second color calibration chart printed by the printing system using a second predetermined ink drop weight that is different from the first ink drop weight. The second color calibration chart includes second colorant ramps including second colorant patches with increasing levels of colorant coverage between adjacent second colorant patches for one or more colorants used in the printing system. The second set of IDWs (e.g., one IDW for each nozzle in the printing system, where a printhead may include one or more nozzles) may be measured using an IDW sensor while printing the second colorant patches of the second color calibration chart.

205 200 At block, the methoddetermines a first set of eIDWs and a second set of eIDWs from the color measurements of the first color calibration chart and the color measurements of the second color calibration chart, respectively. In an example, the first set of eIDWs may be considered to be at a nominal level. The color measurements may be made using a spectrophotometer.

207 200 At block, the methoddetermines a first set of differences of eIDWs (i.e., a first set of ΔeIDWs) between the first set of eIDWs and the second set of eIDWs, respectively, and a mapping between a set of (the first set of IDWs, the first set of eIDWs) and a set of (the second set of IDWs, the second set of eIDWs). In an example, the first set of ΔeIDWs may be determined using various color calibration methods. In an example the mapping between IDW and eIDW may be linear, polynomial, a radial basis function, and so on.

209 200 At block, the methoddetermines a second set of ΔeIDWs after a third printing by the printing system (e.g., using the printing system to print an actual print job) in a similar manner as the first set of ΔeIDWs was determined (e.g., a third set of IDWs are determined for the third printing) and corrects color calibration based on the second set of ΔeIDWs. In an example, the second set of ΔeIDWs may be determined from a difference between the second set of IDWs and the third set of IDWs (e.g., a first set of AIDWs).

Alternatively, a colorant verification of the printing system or a color calibration of the printing system may be initiated based on the second set of ΔeIDWs.

3 FIG. 200 is a flow chart of an example color calibration adjustment methodof the present disclosure.

301 300 301 At block, the methodbegins. At block, a first reference is established for which color calibration may be done on a particular substrate using a first ink drop weight. For example, each nozzle in the printing system may be set to print with a first nozzle firing pulse width. For example, the first reference may be established by a first printing of a first color calibration chart by a printing system (e.g., first colorant ramps including first colorant patches with increasing levels of colorant coverage between adjacent first colorant patches for one or more colorants used in the printing system), measuring a first set of IDWs (e.g., one IDW for each nozzle in the printing system, where a printhead may include one or more nozzles) while printing for the first colorant patches of the first color calibration chart, and measuring colors of the first color calibration chart (e.g., using a spectrophotometer).

303 300 At block, the methodsets the printing system to a second ink drop weight that is different from the first ink drop weight. For example, each nozzle in the printing system may be set to print with a second nozzle firing pulse width that is different from the first nozzle firing pulse width that was used for establishing the first reference.

305 300 At block, the methodestablishes a second reference using the second ink drop weight. For example, the second reference may be established by a second printing for printing a second color calibration chart by the printing system (e.g., second colorant ramps including second colorant patches with increasing levels of colorant coverage between adjacent second colorant patches for one or more colorants used in the printing system), measuring a second set of IDWs while printing the second colorant patches, and measuring colors of the second color calibration chart (e.g., using the spectrophotometer).

307 300 At block, the methoddetermines a first set of eIDWs and a second set of eIDWs from the color measurements of the first color calibration chart and the color measurements of the second color calibration chart, respectively. In an example, the first set of eIDWs may be considered to be at a nominal level.

309 300 At block, the methoddetermines a first set of differences of eIDWs (i.e., a first set of ΔeIDWs) between the first set of eIDWs and the second set of eIDWs, respectively. In an example, the first set of ΔeIDWs may be determined using various color calibration methods.

311 100 At block, the methoddetermines a mapping between a set of (the first set of IDWs, the first set of eIDWs) and a set of (the second set of IDWs, the second set of eIDWs). In an example the mapping between IDW and eIDW may be linear, polynomial, a radial basis function, and so on.

313 300 At block, the methoddetermines a second set of ΔeIDWs after a third printing by the printing system (e.g., using the printing system to print an actual print job) in a similar manner as the first set of ΔeIDWs was determined (e.g., a third set of IDWs are determined for the third printing). In an example, the second set of ΔeIDWs may be determined from a difference between the second set of IDWs and the third set of IDWs (e.g., a first set of AIDWs).

315 300 At block, the methodcorrects color calibration based on the second set of ΔeIDWs. Alternatively, a colorant verification of the printing system or a color calibration of the printing system may be initiated based on the second set of AIDWs.

315 300 In an example, when a color calibration is initiated (e.g., by blockof the methodor by an operator of the printing system), a fourth color calibration chart may be printed, similarly as the first color calibration chart was printed. A fourth set of IDWs may be measured using an IDW sensor while the fourth color calibration chart is printed, similarly as the first set of IDWs was measured. The colors of the fourth color calibration chart may be measured using a spectrophotometer. A fourth set of eIDWs may be determined from the color measurements of the fourth color calibration chart. Color calibration may be performed based on the color measurements of the first color calibration chart and the fourth color calibration chart, where a particular method may be used to perform color calibration. The IDW to eIDW mapping may then be updated based on the fourth sets of IDWs and eIDWs.

In an example, how IDWs and eIDWs relate for different substrate categories may be determined off-line. This may render optional the steps of setting a printing system to a second ink drop weight and establishing a second reference, but may only apply where substrate categories are homogeneous and where the substrate categories may be well sampled in advance (e.g., for aqueous technical or aqueous graphics printing systems, but may not apply for Latex printing systems).

Updating the mapping of IDW to eIDW may be done by different methods. In an example, a mapping of IDW to eIDW may be determined from data concerning the first reference and a subsequent color calibration operation. In an example, the mapping of IDW to eIDW may be done using all available IDW-eIDW data at the current time for a given media. In an example, when all available data is used, the data may be filtered (e.g., outlier removal) or weighted (e.g., more recent data may be given more weight).

Variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.